Methods of denitrating exhaust gases

ABSTRACT

A method of denitrating an exhaust gas by means of removing nitrogen oxides catalytically using ammonia as a reducing agent while introducing an exhaust gas containing the nitrogen oxides into a reaction vessel packed with a catalyst, comprising: 
     providing a denitration catalyst layer in the upstream of the gas flow, an ammonia decomposition catalyst layer capable of decomposing oxidatively ammonia into nitrogen and nitrogen oxides in the downstream and a second denitration catalyst layer or a denitration catalyst layer capable of decomposing ammonia in the further downstream; and, 
     adding ammonia in an amount not less than the reaction equivalence for the nitrogen oxides in the exhaust gas to the inlet of the first denitration catalyst layer.

This is a divisional application of U.S. Ser. No. 08/508,174 filed Jul.27, 1996 now U.S. Pat. No. 5,728,356.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method of denitrating an exhaust gasby which nitrogen oxides (NOx) in the exhaust gas can be eliminated at ahigh efficiency.

DESCRIPTION OF RELATED ART

As a method of eliminating NOx contained in a burning exhaust gas, aselective catalytic hydrogenation using NH₃ as a reducing agent is ofwide use majorly in thermal-power stations. As a catalyst, a titaniumoxide-based catalyst having vanadium, tungsten or molybdenum as anactive component is mainly employed.

Control of NOx emission becomes strict more and more in these days, anda higher efficiency of denitration is required in suburban thermal-powerstations along with the enlargement of generating facilities in responseto the increase in demand of electricity power.

A conventional denitration method employs NH₃ as a reducing agent, anddecomposes NOx into N₂ on the catalyst according to the formula shownbelow.

    4NO+4NH.sub.3 +O.sub.2 →4N.sub.2 +6H.sub.2 O

Based on this formula, it is possible theoretically that 100%elimination of NOx is conducted by adding NH₃ in the molar amount equalto that of NOx. However, it is not practical to effect completelyuniform mixing of NH₃ and NOx in the exhaust gas, and NH₃ should beadded in excess of NOx for the purpose of highly efficient denitration.Accordingly, disadvantage has been experienced that unreacted NH₃ isemitted in a significant amount.

SUMMARY OF THE INVENTION

According to the present invention, highly efficient denitration can beconducted while reducing the emission of unreacted NH₃ into atmosphereto a level as low as possible.

Thus, the present invention provides a first method of denitrating anexhaust gas by means of removing nitrogen oxides catalytically usingammonia as a reducing agent while introducing an exhaust gas containingthe nitrogen oxides into a reaction vessel packed with a catalyst,comprising:

providing a first denitration catalyst layer in the upstream of the gasflow, an ammonia decomposition catalyst layer containing an ammoniadecomposition catalyst capable of decomposing oxidatively ammonia intonitrogen and nitrogen oxides in the downstream and a second denitrationcatalyst layer in the further downstream; and,

adding ammonia in an amount not less than the reaction equivalence forthe nitrogen oxides in the exhaust gas to the inlet of the firstdenitration catalyst layer.

In the first denitration method described above in the presentinvention, the ammonia decomposition catalyst may be a catalyst having aperformance of 70% or higher as % nitrogen selection defined as follows:

% nitrogen selection=[1-{(NOx (ppm) at outlet of ammonia decompositioncatalyst-NOx (ppm) at inlet of ammonia decomposition catalyst)/(NH₃(ppm) at inlet of ammonia decomposition catalyst-NH₃ (ppm) at outlet ofammonia decomposition catalyst)}].

Furthermore, an objective of the present invention is to provide a 2ndmethod of denitrating an exhaust gas by means of removing nitrogenoxides catalytically using ammonia as a reducing agent while introducingan exhaust gas containing the nitrogen oxides into a reaction vesselpacked with a catalyst, comprising:

providing a denitration catalyst layer in the upstream of the gas flow,an ammonia decomposition catalyst layer containing an ammoniadecomposition catalyst capable of decomposing oxidatively ammonia intonitrogen and nitrogen oxides in the downstream and a denitrationcatalyst layer capable of decomposing ammonia in the further downstream;and,

adding ammonia in an amount not less than the reaction equivalence forthe nitrogen oxides in the exhaust gas whereby removing the nitrogenoxides contained in said exhaust gas.

Furthermore, in the first or second denitration method described above,the ammonia decomposition catalyst may be a catalyst having acrystalline silicate which is represented by the formula as dehydrated:

    (1.0±0.8)R.sub.2 O•[aM.sub.2 O.sub.3 •bAl.sub.2 O.sub.3 ]•cMeO•ySiO.sub.2

wherein R denotes an alkaline metal ion and/or hydrogen ion, M denotesat least one element selected from the group consisting of VIII groupelements, rare earth elements, titanium, vanadium, chromium, niobium,antimony and gallium, Me denotes an alkaline earth metal, a+b=1, a≧0,b≧0, c≧0, y/c>12 and y>12, and which has a X-ray diffraction patternshown in Table 1 in the specification as a carrier and at least onemetal selected from the group consisting of platinum, palladium,ruthenium, iridium, and rhodium as an active metal.

                  TABLE 1                                                         ______________________________________                                        Lattice spacing (interstitial distance)                                       (d value)           Relative strength                                         ______________________________________                                        11.2 ± 0.3       VS                                                        10.0 ± 0.3       VS                                                        6.7 ± 0.2        W                                                         6.4 ± 0.2        M                                                         6.0 ± 0.2        M                                                         5.7 ± 0.2        W                                                         5.6 ± 0.2        M                                                         4.6 ± 0.1        W                                                         4.25 ± 0.1       M                                                         3.85 ± 0.1       VS                                                        3.75 ± 0.1       S                                                         3.65 ± 0.1       S                                                         3.3 ± 0.1        M                                                         3.05 ± 0.1       W                                                         3.0 ± 0.1        M                                                         ______________________________________                                         VS: Very strong                                                               S: Strong                                                                     M: Medium                                                                     W: Weak                                                                       (Xray source: Cu)                                                        

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawings inwhich:

FIG. 1 shows a schematic view illustrating a first method (system) ofdenitrating an exhaust gas according to the present invention; and,

FIG. 2 shows a schematic view illustrating a second method (system) ofdenitrating an exhaust gas according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the first denitration method according to the presentinvention is described with referring to FIG. 1. First denitrationcatalyst layer 1001 is provided in the furthermost upstream of the gasflow, and NH₃ decomposition catalyst layer 1002 is provided in thedownstream, and then second denitration catalyst layer 1003 is providedin the further downstream, and NH₃ in an amount not less than thereaction equivalence for the NOx in the exhaust gas is added to theinlet of the first denitration catalyst layer 1001, whereby accomplish90% or more of the denitration reaction in the first denitrationcatalyst layer 1001. Unreacted NH₃ coming from first denitrationcatalyst layer 1001 is decomposed by NH₃ decomposition catalyst layer1002 to adjust the concentrations of NOx and NH₃ at the inlet of seconddenitration catalyst layer 1003 located in the downstream, wherebyreducing the concentrations of NOx and NH₃ at the outlet of seconddenitration catalyst layer 1003 to the levels not exceeding 0.1 ppm and3 ppm, respectively. First and second denitration catalyst layers 1001and 1003 provided in the upstream and downstream, respectively, mayemploy conventional TiO₂ -based catalysts containing V, W or Mo as anactive component.

In the embodiment of the present invention described above, it ispreferable that the catalyst employed in NH₃ decomposition catalystlayer 1002 has a % nitrogen selection as defined below not less than70%.

% nitrogen selection=[1-{(NOx (ppm) at outlet of ammonia decompositioncatalyst-NOx (ppm) at inlet of ammonia decomposition catalyst)/(NH₃(ppm) at inlet of ammonia decomposition catalyst-NH₃ (ppm) at outlet ofammonia decomposition catalyst)}].

Thus, since a small % nitrogen selection of the ammonia decompositioncatalyst as defined above leads to a small range of the operation of thefacility for achieving 0 <NH₃ (ppm)-NOx (ppm)<3 (ppm) at the outlet ofammonia decomposition catalyst layer, it is required to control thefacility with a wide range of gas treatment and temperature.Accordingly, % nitrogen selection is preferably 70% or higher.

As a NH₃ decomposition catalyst having the % nitrogen selectionspecified above, a catalyst having a crystalline silicate which isrepresented by the formula as dehydrated:

    (1.0±0.8)R.sub.2 O•[aM.sub.2 O.sub.3 •bAl.sub.2 O.sub.3 ]•cMeO•ySiO.sub.2

wherein R denotes an alkaline metal ion and/or hydrogen ion, M denotesat least one element selected from the group consisting of VIII groupelements, rare earth elements, titanium, vanadium, chromium, niobium,antimony and gallium, Me denotes an alkaline earth metal, a+b=1, a≧0,b≧0, c≧0, y/c>12 and y>12, and which has a X-ray diffraction patternshown in Table 1 shown above as a carrier and at least one metalselected from the group consisting of platinum, palladium, ruthenium,iridium, and rhodium as an active metal is preferable.

The exhaust gas which has been denitrated by supplying an excessiveamount of NH₃ in first denitration catalyst layer 1001 now has thecomposition of NOx:0 to 10 ppm and NH₃ :10 to 30 ppm, and then isintroduced into the above-mentioned NH₃ decomposition catalyst layerwhereby reducing the amount of NH₃ to achieve the condition "0<NH₃(ppm)-NOx (ppm)<3 (ppm)" at the outlet of NH₃ decomposition catalystlayer 1002, and the exhaust gas having this composition is thendenitrated in second denitration catalyst layer 1003 to achieve theconcentrations of NOx and NH₃ not exceeding 0.1 ppm and 3 ppm,respectively.

Contrary to this, a conventional method wherein NOx in an exhaust gas isdenitrated by using only a denitration catalyst layer while adding anexcessive amount of NH₃ to achieve a concentration of NOx in the exhaustgas not exceeding 0.1 ppm can not avoid a content of NH₃ in the exhaustgas which is at least 10 ppm.

Example 1 described below was conducted in relation with the firstdenitration method referring to FIG. 1.

EXAMPLE 1

(Preparation of denitration catalyst)

A powder catalyst consisting of 4% by weight of vanadium pentaoxide (V₂O₅) and 8% by weight of tungsten trioxide (WO₃) on titanic (T_(i) O₂)was molded into a lattice honeycomb having the pitch of 3.3 mm and thewall thickness of 0.5 mm, which was used as a denitration catalyst.

(Preparation of NH₃ decomposition catalyst)

5616 g of water glass #1 (SiO₂ : 30%) was dissolved in 5429 g of waterto yield solution A. Separately, 718.9 g of aluminum sulfate, 110 g offerric chloride, 47.2 g of calcium acetate, 262 g of sodium chloride and2020 g of concentrated hydrochloric acid were dissolved together in 4175g of water to yield solution B. Solution A and solution B were fed in aconstant ratio to form a precipitate, and the mixture was stirredthoroughly to yield a slurry at pH 8.0. The slurry thus obtained wascharged in a 20 L autoclave, to which 500 g of tetrapropyl ammoniumbromide was added and the mixture was subjected to hydrothermalsynthesis at 160° C. for 72 hours. After synthesis, washing with water,drying and sintering for 3 hours at 500° C, crystalline silicate 1 wasobtained. Crystalline silicate 1 thus obtained is represented in a molarratio (excluding crystal water) by the formula shown below and has thecrystal structure by X-day diffraction pattern in Table 1 shown above.

    0.5Na.sub.2 O•0.5H.sub.2 O•[0.8Al.sub.2 O.sub.3 •0.2Fe.sub.2 O.sub.3 •0.25CaO]•25SiO.sub.2

Crystalline silicate 1 obtained above was subjected to NH₄ ion exchangeby stirring with 4N aqueous solution of NH₄ Cl at 40° C. for 3 hours.After the ion exchange, the silicate was washed and dried at 100° C. for24 hours and sintered at 400° C. for 3 hours to obtain crystallinesilicate 1 of type H. Crystalline silicate of type H is of the typewhich has protons on its surface.

Crystalline silicate 1 of type H thus obtained was impregnated with eachof the aqueous solutions of chloroplatinic acid, palladium nitrate,ruthenium chloride and iridium chloride, evaporated to dryness andsintered at 500° C. for 3 hours to obtain a powder catalyst. To 100 g ofthe powder obtained, 3 g of alumina sol and 55 g of silica sol (SiO₂ :20% by weight) as binders and 200 g of water were added to obtain aslurry, which was wash-coated to a monolith support for coagulate(30-cells/square inch in a form of lattice) at the coating rate of 200g/m². The catalysts thus obtained were designated as NH₃ decompositioncatalysts 1 to 5. The characteristics of the catalysts are shown inTable 2 below.

                  TABLE 2                                                         ______________________________________                                        NH.sub.3 decomposition catalyst No.                                                           Active metal (amount supported: wt %)                         ______________________________________                                        1               Pt          (0.02)                                            2               Pd          (1)                                               3               Ru          (0.3)                                             4               Ir          (0.5)                                             ______________________________________                                    

Crystalline silicates 2 to 8 of type H were prepared similarly in themethod of preparation of NH₃ decomposition catalysts described aboveexcept for using 112 g of cobalt chloride, 105 g of titanium chloride,10 g of vanadium chloride, 107 g of chromium chloride, 135 g of niobiumchloride, 155 g of antimony chloride and 119 g of gallium chlorideinstead of ferric chloride. Using these crystalline silicates of type Hand the aqueous solution of chloroplatinic acid, platinum was supportedon each crystalline silicate of type H and wash-coated to a monolithsupport for coagulate similarly as described above at the coating rateof 200 g/m² of support surface. The catalysts thus obtained weredesignated as NH₃ decomposition catalysts 6 to 11. The characteristicsof the catalysts are shown in Table 3 below.

                  TABLE 3                                                         ______________________________________                                        NH.sub.3 decomposition catalyst No.                                                           Active metal (amount supported: wt %)                         ______________________________________                                        5               Pt (0.02)                                                     6               Pt (0.02)                                                     7               Pt (0.02)                                                     8               Pt (0.02)                                                     9               Pt (0.02)                                                     10              Pt (0.02)                                                     11              Pt (0.02)                                                     ______________________________________                                    

(Denitration reaction test: Test No.1)

Three denitration catalysts having the size of 40 mm×50 mm×400 mmL, 1NH₃ decomposition catalyst having the size of 42 mm×50 mm×500 mmL and 2denitration catalysts, all described above, were attached serially andtested in the condition described below.

                  TABLE 4                                                         ______________________________________                                        Concentration of NOx in exhaust gas                                                                  50 ppm                                                 Concentration of NH.sub.3 supplied                                                                   60 ppm, 70 ppm                                         Concentration of O.sub.2 in exhaust gas                                                              14.7%                                                  Gas flow rate          22 Nm.sup.3 /hour                                      Gas temperature        360° C.                                         ______________________________________                                    

The results are shown in Table 5. At the outlet of the upstream firstdenitration catalyst layer which corresponds to a conventionaldenitration method, NOx was reduced to 0.02 to 0.03 ppm which indicateda high denitration efficiency, but NH₃ was emitted at a level as high as10 to 20 ppm. In contrast, the method according to the present inventionprovided the levels of NOx and NH₃ at the outlet of the downstreamsecond denitration catalyst layer as low as 0.02 to 0.1 ppm and lessthan 3 ppm, respectively, ensuring the low level of emission of both ofNOx and NH₃.

                  TABLE 5                                                         ______________________________________                                                           First        Second                                                           denitration catalyst                                                                       denitration catalyst                          NH.sub.3           layer outlet layer outlet                                  decomposition                                                                           Inlet NH.sub.3                                                                         NOx      NH.sub.3                                                                            NOx   NH.sub.3                              catalyst No.                                                                            (ppm)    (ppm)    (ppm) (ppm) (ppm)                                 ______________________________________                                        1         60       0.02     10.1  0.02  0.4                                             70       0.02     19.8  0.02  1.0                                   2         60       0.02     10.2  0.03  1.5                                             70       0.02     20.0  0.05  2.3                                   3         60       0.02     10.1  0.1   1.0                                             70       0.02     19.7  0.07  2.2                                   4         60       0.02     9.9   0.03  1.0                                             70       0.02     20.1  0.02  1.8                                   5         60       0.02     10.0  0.05  2.0                                             70       0.02     20.1  0.07  2.6                                   6         60       0.02     10.1  0.02  1.5                                             70       0.02     19.9  0.03  2.4                                   7         60       0.02     9.9   0.02  2.2                                             70       0.02     20.2  0.02  2.9                                   8         60       0.02     10.2  0.03  1.3                                             70       0.02     19.8  0.03  2.0                                   9         60       0.02     10.0  0.05  1.2                                             70       0.02     20.0  0.04  2.1                                   10        60       0.02     10.2  0.03  1.6                                             70       0.02     20.1  0.04  2.3                                   11        60       0.02     10.2  0.1   2.0                                             70       0.02     19.9  0.06  2.8                                   ______________________________________                                    

An embodiment of the second denitration method according to the presentinvention is described with referring to FIG. 2. As shown in FIG. 2,first denitration catalyst layer 2001 is provided in the furthermostupstream of the gas flow, and NH₃ decomposition catalyst layer 2002 isprovided in the downstream, and then denitration catalyst layer 2003having NH₃ decomposition ability is provided in the further downstream.NH₃ in an amount not less than the reaction equivalence for the NOx isadded to the upstream to accomplish 90% or more of the denitrationreaction in the upstream denitration catalyst layer. Then NH₃ isdecomposed by NH₃ decomposition catalyst layer 2002 to adjust theconcentrations of NOx and NH₃ at the inlet of the downstream denitrationcatalyst layer 2003 having NH₃ decomposition ability, whereby reducingthe concentrations of NOx and NH₃ at the final outlet to the levels notexceeding 0.1 ppm and 3 ppm, respectively.

The upstream denitration catalyst employed TiO₂ catalysts having V, W orMo as an active component and supported 4.5% by weight of V₂ O₅, whilethe downstream denitration catalyst having NH₃ decomposition abilitysupported V₂ O₅ in an amount greater than that supported by the upstreamcatalyst, i.e., in an amount of 4.5% by weight or greater, or, contained0.0001% by weight or more of a noble metal (Pt, Pd, Ru, Rh or Ir).

As a NH₃ decomposition catalyst, a catalyst having a crystallinesilicate which is represented by the formula as dehydrated:

    (1±0.8)R.sub.2 O•[aM.sub.2 O.sub.3 •bAl.sub.2 O.sub.3 ]•cMeO•ySiO.sub.2

wherein R denotes an alkaline metal ion and/or hydrogen ion, M denotesat least one element selected from the group consisting of VIII groupelements, rare earth elements, titanium, vanadium, chromium, niobium,antimony and gallium, Me denotes an alkaline earth metal, a+b=1, a≧0,b≧0, c≧0, y/c>1² and y>12, and which has a X-ray diffraction patternshown in Table 1 shown above as a carrier and at least one metalselected from the group consisting of platinum, palladium, ruthenium andiridium as an active metal may be employed.

It is preferable to control the operation to achieve the concentrationsof NOx and NH₃ of 0 to 10 ppm and 10 to 30 ppm, respectively, at theoutlet of the upstream denitration catalyst layer, and to reduce NH₃ inthe subsequent NH₃ decomposition catalyst layer to achieve thedifference between the concentrations of NH₃ and NOx at its outlet:0<NH₃ (ppm)-NOx (ppm)<3 ppm. By such denitration method, theconcentrations of NOx and NH₃ not exceeding 0.1 ppm and 3 ppm,respectively, at the outlet can be achieved.

In the conventional denitration methods employing only denitrationcatalyst layers, 10 ppm or higher of NH₃ emission could not be avoidedin order to achieve 0.1 ppm or lower of NOx at the outlet.

Example 2 described below was conducted in relation with the seconddenitration method referring to FIG. 2.

EXAMPLE 2

(Preparation of denitration catalyst 101)

A powder catalyst consisting of 4% by weight of vanadium pentaoxide (V₂O₅) and 8% by weight of tungsten trioxide (WO₃) on titanic (TiO₂) wasmolded into a lattice honeycomb having the pitch of 3.3 mm and the wallthickness of 0.5 mm, which was designated as denitration catalyst 101.

(Preparation of NH₃ decomposition catalysts 101 to 119)

The 5616 g of water glass #1 (SiO₂ : 30%) was dissolved in 5429 g ofwater to yield solution A. Separately, 718.9 g of aluminum sulfate, 110g of ferric chloride, 47.2 g of calcium acetate, 262 g of sodiumchloride and 2020 g of concentrated hydrochloric acid were dissolvedtogether in 4175 g of water to yield solution B. Solution A and solutionB were fed in a constant ratio to form a precipitate, and the mixturewas stirred thoroughly to yield a slurry at pH 8.0. The slurry thusobtained was charged in a 20 L autoclave, to which 500 g of tetrapropylammonium bromide was added and the mixture was subjected to hydrothermalsynthesis at 160° C. for 72 hours.

After synthesis, washing with water, drying and sintering for 3 hours at500° C., crystalline silicate 101 was obtained. Crystalline silicate 101thus obtained is represented in a molar ratio (excluding crystal water)by the formula shown below and has the crystal structure by X-laydiffraction pattern in Table 1 shown above.

    0.5Na.sub.2 O•0.5H.sub.2 O•[0.8Al.sub.2 O.sub.3 •O0.2Fe.sub.2 O.sub.3 O•0.25CaO]•25SiO.sub.2

Crystalline silicate 101 obtained above was subjected to NH₄ ionexchange by stirring with 4N aqueous solution of NH₄ Cl at 40° C. for 3hours. After the ion exchange, the silicate was washed and dried at 100°C. for 24 hours and sintered at 400° C. for 3 hours to obtaincrystalline silicate 101 of type H (a catalyst which has protons on itssurface).

Crystalline silicate 101 of type H thus obtained was impregnated witheach of the aqueous solutions of chloroplatinic acid, palladium nitrate,ruthenium chloride, chloroiridic acid and rhodium chloride, evaporatedto dryness and sintered at 500° C. for 3 hours to obtain a powdercatalyst.

To 100 g of the powder obtained, 3 g of alumina sol and 55 g of silicasol (SiO₂ : 20% by weight) as binders and 200 g of water were added toobtain a slurry, which was wash-coated to a monolith support forcoagulate (30-cells/square inch in a form of lattice) at the coatingrate of 200 g/m . The catalysts thus obtained were designated as NH₃decomposition catalysts 101 to 105.

Except for adding cobalt chloride, ruthenium chloride, rhodium chloride,lanthanum chloride, cerium chloride, titanium chloride, vanadiumchloride, chromium chloride, antimony chloride, gallium chloride andniobium chloride in the molar amount as an oxide equal to that of Fe₂ O₃instead of ferric chloride used in the synthesis of crystalline silicate101 in the preparation of NH₃ decomposition catalysts 101 to 105described above, the procedure similar to that employed for crystallinesilicate 101 was conducted to obtain crystalline silicates 102 to 112.The crystal structures of these crystalline silicates as X-raydiffraction patterns were shown above in Table 1, with the compositionsbeing represented by the following formula in molar ratios of the oxides(dehydrated form): 0.5Na₂ O•0.5H₂ O•(0.2M₂ O₃ •0.8Al₂ O₃•0.25CaO)•25SiO₂, wherein M denotes Co, Ru, Rh, La, Ce, Ti, V, Cr, Sb,Ga or Nb.

Then, except for adding magnesium acetate, strontium acetate and bariumacetate each in the molar amount as an oxide equal to that of CaOinstead of calcium acetate used in the synthesis of crystalline silicate101, the procedure similar to that employed for crystalline silicate 101was conducted to obtain crystalline silicates 113 to 115. The crystalstructures of these crystalline silicates as X-ray diffraction patternswere shown above in Table 1, with the compositions being represented bythe following formula in molar ratios of the oxides (dehydrated form):0.5Na₂ O•0.5H₂ O•(0.2Fe₂ O₃ •0.8Al₂ O₃ •0.25MeO)•25SiO₂, where Medenotes Mg, Sr or Ba.

Using crystalline silicates 102 to 115 obtained above and the proceduresimilar to that for crystalline silicate 101 (the procedure describedabove), crystalline silicates 102 to 115 of type H were obtained. Thesecrystalline silicates of type H were impregnated with the aqueoussolution of chloroplatinic acid, evaporated to dryness and sintered at500° C. for 3 hours to obtain powder catalysts containing 0.02% byweight of Pt. These powder catalysts were coated to a monolith supportfor coagulate similarly as for decomposition catalysts 101 to 105,whereby obtaining NH₃ decomposition catalysts 106 to 119. Thecharacteristics of NH₃ decomposition catalysts 101 to 119 are shown inTable 6 below.

                                      TABLE 6                                     __________________________________________________________________________    NH.sub.3 decomposition                                                                 Wt % Active metal (value in                                                                Type H crystalline silicate                             catalyst No.                                                                           bracket: amount supported)                                                                 No.                                                                              Composition                                          __________________________________________________________________________    101      Pt (0.02)    101                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Fe.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       102      Pd (1)       101                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Fe.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       103      Ru (0.3)     101                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Fe.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       104      Ir (0.5)     101                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Fe.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       105      Rh (0.1)     101                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Fe.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       106      Pt (0.02)    102                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Co.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       107      Pt (0.02)    103                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Ru.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       108      Pt (0.02)    104                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Rh.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       109      Pt (0.02)    105                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 La.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       110      Pt (0.02)    106                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Ce.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       111      Pt (0.02)    107                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Ti.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       112      Pt (0.02)    108                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 V.sub.2                                   O.sub.3.0.25 CaO).25 SiO.sub.2                       113      Pt (0.02)    109                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Cr.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       114      Pt (0.02)    110                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Sb.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       115      Pt (0.02)    111                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Ga.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       116      Pt (0.02)    112                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Nb.sub.2                                  O.sub.3.0.25 CaO).25 SiO.sub.2                       117      Pt (0.02)    113                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Fe.sub.2                                  O.sub.3.0.25 MgO).25 SiO.sub.2                       118      Pt (0.02)    114                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Fe.sub.2                                  O.sub.3.0.25 SrO).25 SiO.sub.2                       119      Pt (0.02)    115                                                                              H.sub.2 O.(0.8 Al.sub.2 O.sub.3.0.2 Fe.sub.2                                  O.sub.3.0.25 BaO).25 SiO.sub.2                       __________________________________________________________________________

(Preparation of denitration catalysts 201 to 203 having NH₃decomposition ability)

A powder catalyst consisting of 6% by weight of vanadium pentaoxide (V₂O₅) and 9% by weight of tungsten trioxide (WO₃) on titanic (TiO₂) wasmolded into a lattice honeycomb having the pitch of 3.3 mm and the wallthickness of 0.5 mm, which was designated as denitration catalyst 201having NH₃ decomposition ability.

A powder catalyst supporting 9% by weight of molybdenum trioxide (MoO₃)instead of tungsten trioxide (WO₃) employed in the preparation ofdenitration catalyst 201 having NH₃ decomposition ability describedabove was prepared by the method similar to that described above toobtain denitration catalyst 202 having NH₃ decomposition ability.

Furthermore, a powder catalyst supporting 3% by weight of vanadiumpentaoxide and 0.001% by weight of platinum instead of 6% by weight ofvanadium pentaoxide (V₂ O₅) employed in the preparation of denitrationcatalyst 201 having NH₃ decomposition ability described above wasprepared by the method similar to that described above to obtaindenitration catalyst 203 having NH₃ decomposition ability. (Denitrationreaction test: Test No.2-1) Three denitration catalysts 101 having thesize of 40 mm×50 mm×400 mmL, 1 NH₃ decomposition catalyst 101 having thesize of 42 mm×50 mm×150 mmL and 2 denitration catalysts 201 having NH₃decomposition ability which had the size of 40 mm×50 mm×400 mmL were allattached serially and subjected as system 301 to the denitrationreaction test in the condition indicated in Table 7 shown below.

                  TABLE 7                                                         ______________________________________                                        Exhaust gas composition                                                                         Gas amount                                                                              Temperature                                       ______________________________________                                        Inlet NO: 50 ppm  22 Nm.sup.3 /h                                                                          360° C.                                    Inlet NH.sub.3 : 60 ppm, 80 ppm                                               Inlet O.sub.2 : 14.7%                                                         Inlet H.sub.2 O: 6%                                                           ______________________________________                                    

(Denitration reaction test: Test No.2-2)

Systems 302 to 319 were obtained by providing NH₃ decompositioncatalysts 102 to 119 instead of NH₃ decomposition catalyst 1 employed inTest No.2-1, and subjected to the denitration reaction test in thecondition similar to that in Test No.2-1.

(Denitration reaction test: Test No.2-3)

Systems 320 and 321 were obtained by providing denitration catalysts 202and 203 having NH₃ decomposition ability instead of denitration catalyst201 having NH₃ decomposition ability employed in Test No.2-1, andsubjected to the denitration reaction test in the condition similar tothat in Test No.2-1. The results of the tests of systems 301 to 321 areshown in Table 8.

(Comparative denitration reaction test No.1)

In a comparative test, system 322 was obtained by providing onlydenitration catalyst 101 employed in Test No.2-1 and subjected to thedenitration performance test similarly as in Test No.2-1.

In addition, system 323 was obtained by providing only denitrationcatalyst 101 and NH₃ decomposition catalyst 101 employed in Test No.2-1and subjected to the denitration performance test similarly as in TestNo.2-1.

Furthermore, system 324 was obtained by providing two denitrationcatalysts 101 identical to the first layer in Test No.2-1 instead ofdenitration catalyst 201 having NH₃ decomposition ability provided asthe third layer in Test No.2-1, and subjected to the denitrationperformance test similarly as in Test No.2-1. The results of the testsof systems 322 to 324 are shown also in Table 8.

As evident from the results shown in Table 8, systems 301 to 321according to the method of the present invention achieved aconcentration of NOx at the outlet of the third catalyst layer of 0.02to 0.04 ppm, indicating a very high denitration efficiency, whilereducing the leak of NH₃ to a level as low as 0.7 to 2.3 ppm.

On the other hand, system 322 corresponding to a conventionaldenitration method involved the problem of the NH₃ emission as high as10 to 30 ppm although it achieved a NOx level of 0.02 to 0.03 ppm, asshown in Comparative test No.1.

System 323 wherein only NH₃ decomposition catalyst was provided as thesecond layer provided the concentrations of NOx and NH₃ ranging from 0.7to 1.6 ppm and 2.4 to 7.8 ppm, respectively, which did not indicate ahigh denitration efficiency and were not NOx emission levelcorresponding to an atmospheric level. Furthermore, system 324 whereinthe denitration catalyst having no NH₃ decomposition ability identicalto the first layer was provided as the third layer provided the NOxlevel at the outlet ranging from 0.02 to 0.03 ppm, but it provided a NH₃level at the outlet ranging from 1.5 to 6.2 ppm, indicating that itinvolved the disadvantage of the leak of NH₃ exceeding 5 ppm in case ofthe NH₃ level at the inlet as high as 80 ppm.

                                      TABLE 8                                     __________________________________________________________________________                        (Third step) Third step                                                (Second step)                                                                        Denitration catalyst                                                                       catalyst layer                                      (First step)                                                                        NH.sub.3                                                                             having NH.sub.3                                                                        Inlet                                                                             outlet                                       Test                                                                             System                                                                            Denitration                                                                         decomposition                                                                        decomposition                                                                          NH.sub.3                                                                          NO.sub.x                                                                          NH.sub.3                                 No.                                                                              No. catalyst No.                                                                        catalyst No.                                                                         ability No.                                                                            (ppm)                                                                             (ppm)                                                                             (ppm)                                    __________________________________________________________________________    2-1                                                                              301 101   101    201      60  0.02                                                                              0.7                                                                   80  0.02                                                                              2.1                                      2-2                                                                              302 "     102    "        60  0.03                                                                              0.8                                                                   80  0.03                                                                              1.9                                      "  303 "     103    "        60  0.02                                                                              0.9                                                                   80  0.02                                                                              2.2                                      "  304 "     104    "        60  0.03                                                                              1.0                                                                   80  0.03                                                                              1.9                                      "  305 "     105    "        60  0.03                                                                              1.1                                                                   80  0.03                                                                              2.0                                      "  306 "     106    "        60  0.03                                                                              0.9                                                                   80  0.02                                                                              1.8                                      "  307 "     107    "        60  0.03                                                                              1.0                                                                   80  0.02                                                                              1.7                                      "  308 "     108    "        60  0.02                                                                              0.9                                                                   80  0.04                                                                              1.8                                      "  309 "     109    "        60  0.03                                                                              1.0                                                                   80  0.04                                                                              1.8                                      "  310 "     110    "        60  0.03                                                                              1.0                                                                   80  0.04                                                                              1.7                                      "  311 "     111    "        60  0.03                                                                              0.9                                                                   80  0.03                                                                              1.9                                      "  312 "     112    "        60  0.03                                                                              1.0                                                                   80  0.04                                                                              2.0                                      2-2                                                                              313 101    13    201      60  0.03                                                                              1.0                                                                   80  0.02                                                                              2.1                                      "  314 "      14    "        60  0.02                                                                              1.1                                                                   80  0.02                                                                              1.9                                      "  315 "      15    "        60  0.04                                                                              1.2                                                                   80  0.02                                                                              2.1                                      "  316 "      16    "        60  0.04                                                                              1.1                                                                   80  0.03                                                                              2.0                                      "  317 "      17    "        60  0.03                                                                              1.0                                                                   80  0.03                                                                              1.9                                      "  318 "      18    "        60  0.03                                                                              0.9                                                                   80  0.04                                                                              2.0                                      "  319 "      19    "        60  0.03                                                                              1.0                                                                   80  0.03                                                                              1.9                                      2-3                                                                              320 "      1     202      60  0.03                                                                              0.8                                                                   80  0.02                                                                              2.2                                      "  321 "      1     203      60  0.03                                                                              0.7                                                                   80  0.03                                                                              2.3                                      1  322 101   --     --       60  0.02                                                                              10.1                                                                  80  0.03                                                                              29.7                                     "  323 101   101    --       60  0.7 2.4                                                                   80  1.6 7.8                                      "  324 101   101    Denitration                                                                            60  0.02                                                                              1.5                                                          catalyst similar                                                                       80  0.03                                                                              6.2                                                          to that in 1st                                                                step 101                                                  __________________________________________________________________________

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method of denitrating an exhaust gas containingnitrogen oxides by means of catalytically removing said nitrogen oxidesusing ammonia as a reducing agent while introducing said exhaust gasinto a reaction vessel packed with a catalyst, comprising:passing saidexhaust gas catalyst through a) a first denitration catalyst layer inthe upstream of the gas flow under conditions effective to reduce saidnitrogen oxides, then b) through an ammonia decomposition catalyst layercontaining an ammonia decomposition catalyst under conditions effectiveto decompose ammonia into nitrogen and nitrogen oxides and then c)through a second denitration catalyst layer downstream from the secondlayer, wherein ammonia is added in an amount not less than thestoichiometric equivalent of the nitrogen oxides in the exhaust gas atthe inlet of the first denitration catalyst layer, and wherein theammonia decomposition catalyst is a catalyst having a performance of 70%or higher as % nitrogen selection defined as follows:% nitrogenselection=[1-(NOx (ppm) at outlet of ammonia decomposition catalyst-NOx(ppm) at inlet of ammonia decomposition catalyst/NH₃ (ppm) at inlet ofammonia decomposition catalyst-NH₃ (ppm) at outlet of ammoniadecomposition catalyst)]×100.
 2. A method of denitrating an exhaust gasaccording to claim 1, wherein the ammonia decomposition catalyst is onecomprising a crystalline silicate of the formula as dehydrated:

    (1.0±0.8)R.sub.2 O•[aM.sub.2 O.sub.3 •bAl.sub.2 O.sub.3 ]•cMeO•ySiO.sub.2

wherein R denotes an alkali metal ion and/or hydrogen ion, M denotes atleast one element selected from the group consisting of VIII groupelements, rare earth elements, titanium, vanadium, chromium, niobium,antimony and gallium, Me denotes an alkaline earth metal, a+b=1,a≧0,b≧0, c=0, and y>12, and which has a X-ray diffraction pattern shownin Table 1 in the specification as a carrier and at least one metalselected from the group consisting of platinum, palladium, ruthenium,iridium, and rhodium as an active metal.
 3. A method of denitrating anexhaust gas containing nitrogen oxides by catalytically removing saidnitrogen oxides using ammonia as a reducing agent while introducing saidexhaust gas containing the nitrogen oxides into a reaction vessel packedwith a catalyst, comprising:passing said exhaust gas catalyst through a)a first denitration catalyst layer in the upstream of the gas flow underconditions effective to reduce said nitrogen oxides, then b) through anammonia decomposition catalyst layer containing an ammonia decompositioncatalyst under conditions effective to decompose ammonia into nitrogenand nitrogen oxides and then c) through a second denitration catalystlayer capable of decomposing ammonia downstream from the second layer,wherein ammonia is added in an amount not less than the stoichiometricequivalent of the nitrogen oxides in the exhaust gas thereby removingthe nitrogen oxides contained in said gas, and wherein the ammoniadecomposition catalyst is a catalyst having a performance of 70% orhigher as % nitrogen selection defined as follows:% nitrogenselection=[1-(NOx (ppm) at outlet of ammonia decomposition catalyst-NOx(ppm) at inlet of ammonia decomposition catalyst/NH₃ (ppm) at inlet ofammonia decomposition catalyst NH₃ (ppm) at outlet of ammoniadecomposition catalyst)]×100.
 4. A method of denitrating an exhaust gasaccording to claim 3, wherein the ammonia decomposition catalyst is onecomprising a crystalline silicate which is represented by the formula asdehydrated:

    (1.0±0.8)R.sub.2 O•[aM.sub.2 O.sub.3 •bAl.sub.2 O.sub.3 ]•cMeO•ySiO.sub.2

wherein R denotes an alkali metal ion and/or hydrogen ion, M denotes atleast one element selected from the group consisting of VIII groupelements, rare earth elements, titanium, vanadium, chromium, niobium,antimony and gallium, Me denotes an alkaline earth metal, a+b=1,a>0,b≧0, c=0, and y>12, and which has a X-ray diffraction pattern shownin Table 1 in the specification as a carrier and at least one metalselected from the group consisting of platinum, palladium, ruthenium,iridium, and rhodium as an active metal.